Constant current and voltage controller in a three-pin package with dual-use switch pin

ABSTRACT

A flyback converter includes a controller integrated circuit (IC) housed in an IC package with only three terminals. The controller IC is grounded through a ground terminal. A feedback signal is received onto a power terminal. The feedback signal powers the controller IC and is derived from a voltage across an auxiliary inductor of the flyback converter. A switch terminal is coupled to an inductor switch that is turned on by a switch control signal having a frequency and a pulse width. The inductor switch controls the current that flows through a primary inductor of the flyback converter. A switch signal is received onto the switch terminal and is used to generate the inductor switch control signal. The controller IC adjusts the frequency in a constant current mode such that output current remains constant and adjusts the pulse width in a constant voltage mode such that output voltage remains constant.

TECHNICAL FIELD

The present invention relates generally to the field of power conversionand, more particularly, to a switch mode power supply circuit thatregulates output current and output voltage using only three pins.

BACKGROUND

Various integrated circuit chips are currently used to control flybackconverters that supply constant current and constant voltage. FIG. 1(prior art) illustrates an exemplary prior art constant output currentflyback converter 10 controlled on the primary side of a transformer 11.Although flyback converter 10 avoids the cost of an opto-couplertypically used in secondary-side controlled converters, flybackconverter 10 requires a relatively expensive integrated circuit package.Flyback converter 10 includes an integrated circuit package with sixpins to accommodate a conventional peak-current-mode pulse widthmodulation (PWM) controller integrated circuit (IC) 12. Typically, ICpackages with more pins are more expensive than IC packages with fewerpins.

In addition, the discrete external components of flyback converter 10also contribute to the manufacturing cost. The external componentsinclude transformer 11, a voltage divider resistor network 13, a primaryswitch 14, a primary-side rectifier 15, a secondary-side rectifier 16and other resistors and capacitors. Transformer 11 has three windings: aprimary-side winding Lp, a secondary-side winding Ls, and an auxiliarywinding La. Primary switch 14 is an external metal-oxide semiconductorfield-effect transistor (MOSFET). A resistor 17 on the secondary side inFIG. 1 represents the resistive loss of the copper windings oftransformer 11. Flyback converter 10 also includes a current senseresistor 18, an output capacitor 19, a start-up resistor 20 and a powercapacitor 21. The initial start-up energy for controller IC 12 isprovided by resistor 20 and capacitor 21. Once flyback converter 10 isstable, auxiliary winding La of transformer 11 powers IC 12 viaprimary-side rectifier 15.

FIG. 2 (prior art) illustrates another constant output current flybackconverter 22 controlled on the primary side of transformer 11. Flybackconverter 22 includes a controller IC 23 that is contained in a four-pinintegrated circuit package. The IC package of flyback converter 22 isless expensive than the IC package of flyback converter 10 becausecontroller IC 23 requires four as opposed to six pins. Flyback converter22 still includes voltage divider resistor network 13. Controller IC 23receives a feedback signal 24 from auxiliary winding La via a voltagedivider resistor network 13 and uses feedback signal 24 to control anexternal NPN bipolar transistor 25. For additional details on a constantoutput current flyback converter that can be packaged in a four-pin ICpackage, see U.S. patent application Ser. No. 11/888,599 entitled“Constant Current and Voltage Controller in a Four-Pin Package withDual-Use Pin,” filed on Jul. 31, 2007, now U.S. Pat. No. 7,522,431,which is incorporated herein by reference.

A less expensive flyback converter is sought that has fewer externalcomponents and that is controlled by a controller IC that is containedin an IC package with fewer pins.

SUMMARY

A flyback converter includes a controller integrated circuit (IC) housedin an IC package with only three terminals: a ground terminal, a powerterminal and a switch terminal. The power terminal is used for multiplefunctions. The controller IC is grounded through the ground terminal.The switch terminal is coupled to an inductor switch that is turned onby an inductor switch control signal having a frequency and a pulsewidth. The inductor switch controls the current that flows through aprimary inductor of the flyback converter. The power terminal receives afeedback signal that is derived from a voltage across an auxiliaryinductor of the flyback converter. The feedback signal provides power tothe controller IC and is also used to generate the inductor switchcontrol signal. The controller IC adjusts the frequency of the inductorswitch control signal in a constant current mode such that the outputcurrent of the flyback converter remains constant. In a constant voltagemode, the controller IC adjusts the pulse width of the inductor switchcontrol signal such that the output voltage remains constant.

The switch terminal receives a switch signal that is indicative of theinductor current flowing through the primary inductor. Controller ICcontrols the pulse width of inductor switch control signal such that thetime at which the inductor current stops increasing through the primaryinductor corresponds to the time at which the switch signal reaches apeak current limit. Controlling the pulse width prevents the outputcurrent from exceeding a predetermined current limit.

The controller IC has a power bond pad, a switch bond pad and a groundbond pad. The power bond pad is coupled to the power terminal; theswitch bond pad is coupled to the switch terminal; and the ground bondpad is coupled to the ground terminal. In one embodiment, the controllerIC has no bond pads other than the power bond pad, the switch bond padand the ground bond pad.

A method of operating a power converter includes a step of magneticallycoupling an auxiliary inductor to a primary inductor and to a secondaryinductor of the power converter. The power converter has an externalinductor switch and a controller IC. The controller IC has an internalinductor switch that is coupled to the external inductor switch. Theinternal inductor switch is turned on and off by an inductor switchcontrol signal. The inductor switch control signal has a frequency and apulse width.

In another step, a feedback signal is derived from a voltage across theauxiliary inductor and is received onto a power bond pad of thecontroller IC. In addition to the power bond pad, the controller IC hasa switch bond pad and ground bond pad. The controller IC is contained inan IC package that has a power terminal, a switch terminal and a groundterminal. The IC package includes no terminals other than the powerterminal, the ground terminal and the switch terminal. The powerterminal is coupled to the power bond pad; the switch terminal iscoupled to the switch bond pad; and the ground terminal is coupled tothe ground bond pad.

In another step, the inductor switch control signal is generated usingthe feedback signal.

In another step, the internal inductor switch is turned on and off usingthe inductor switch control signal.

In another step, the frequency of the inductor switch control signal isadjusted using the feedback signal such that the output current of thepower converter remains constant. Information conveyed in the feedbacksignal while the internal inductor switch is turned off is used toadjust the frequency such that the output current remains constant.

In another step, the pulse width of the inductor switch control signalis adjusted using the feedback signal such that the output voltage ofthe power converter remains constant. Information conveyed in thefeedback signal while the internal inductor switch is turned off is usedto adjust the pulse width such that the output voltage remains constant.

In another embodiment, a power converter includes a primary inductor anda secondary inductor that are magnetically coupled to an auxiliaryinductor. A feedback signal is derived from a voltage across theauxiliary inductor. The power converter also includes a controller ICwith a switch bond pad that is coupled to an inductor switch of thecontroller IC. The inductor switch is turned on and off by an inductorswitch control signal. The power converter also includes a means forreceiving the feedback signal. The feedback signal is used both to powerthe controller IC and to generate the inductor switch control signal.The controller IC uses the feedback signal to adjust the frequency ofthe inductor switch control signal such that the output current of thepower converter remains constant. The controller IC also uses thefeedback signal to adjust the pulse width of the inductor switch controlsignal such that the output voltage of the flyback converter remainsconstant. The controller IC is packaged in an IC package that includesno more than three terminals.

In another embodiment, a flyback converter includes a controller IChoused in an IC package with only three terminals: a ground terminal, apower terminal and a switch terminal. The switch terminal is used formultiple functions. The controller IC is grounded through the groundterminal. An auxiliary voltage signal is received onto the powerterminal and provides power to the controller IC. The auxiliary voltagesignal is derived from a voltage across a first auxiliary inductor ofthe flyback converter. The switch terminal is coupled to an inductorswitch that is turned on and off by an inductor switch control signalhaving a frequency and a pulse width. The inductor switch controls thecurrent that flows through a primary inductor of the flyback converter.The inductor switch is coupled through an external transistor to theprimary inductor. A switch signal is received onto the switch terminaland is used to generate the inductor switch control signal. The switchsignal provides information that allows the flyback converter to outputa constant current during a constant current mode, to output a constantvoltage during a constant voltage mode and to prevent the output currentfrom exceeding a predetermined current limit. Information conveyed inthe switch signal provides an indication both of the output voltage ofthe flyback converter and of when the current has stopped increasing inmagnitude through the primary inductor.

The controller IC uses the information from the switch signal togenerate the inductor switch control signal in both the constant currentmode and in the constant voltage mode. The controller IC adjusts thefrequency of the inductor switch control signal in the constant currentmode such that the output current remains constant and adjusts the pulsewidth of the inductor switch control signal in the constant voltage modesuch that the output voltage remains constant. The controller IC alsouses the information from the switch signal to control the peak currentthat flows through the primary inductor such that the output current ofthe flyback converter does not exceed the predetermined current limit.

In another embodiment, a power converter has a controller IC, a primaryinductor, a secondary inductor, a first auxiliary inductor and a secondauxiliary inductor. The auxiliary inductors are magnetically coupled tothe primary and secondary inductors. The controller IC has an inductorswitch, a power bond pad, a switch bond pad and a ground bond pad. Thecontroller IC receives power through the power bond pad and is groundedthrough the ground bond pad. The inductor switch is coupled to theswitch bond pad and is turned on and off by an inductor switch controlsignal. The switch bond pad receives a switch signal that is used by thecontroller IC to generate the inductor switch control signal. Thecontroller IC uses the switch signal to adjust the frequency of theinductor switch control signal in a constant current mode such that theoutput current of the power converter remains constant. The controllerIC also uses the switch signal to adjust the pulse width of the inductorswitch control signal in a constant voltage mode such that the outputvoltage of the power converter remains constant. The controller IC alsouses the switch signal to adjust the pulse width of the inductor switchcontrol signal such that the output current of the flyback converterdoes not exceed a predetermined current limit.

A method of operating a power converter includes a step of magneticallycoupling an auxiliary inductor to a primary inductor and to a secondaryinductor of the power converter. The power converter has an externalinductor switch and a controller IC. The controller IC has an internalinductor switch that is coupled to the external inductor switch. Theinternal inductor switch is turned on and off by an inductor switchcontrol signal. The inductor switch control signal has a frequency and apulse width.

In another step, a switch signal is received onto a switch bond pad ofthe controller integrated circuit. The switch signal is derived from avoltage across the auxiliary inductor.

In another step, the inductor switch control signal is generated usingthe switch signal.

In another step, the internal inductor switch is turned on and off usingthe inductor switch control signal.

In another step, the frequency of the inductor switch control signal isadjusted using the switch signal such that the output current of thepower converter remains constant. Information conveyed in the switchsignal while the internal inductor switch is turned off is used toadjust the frequency such that the output current remains constant.

In another step, the pulse width of the inductor switch control signalis adjusted using the switch signal such that the output voltage of thepower converter remains constant. Information conveyed in the switchsignal while the internal inductor switch is turned off is used toadjust the pulse width such that the output voltage remains constant.

In another embodiment, a power converter includes a primary inductorthat is magnetically coupled to a first auxiliary inductor and to asecond auxiliary inductor. An auxiliary voltage signal is derived from avoltage across the first auxiliary inductor. The power converter alsoincludes a controller IC with a switch bond pad that is coupled to aninductor switch of the controller IC. The inductor switch is turned onand off by an inductor switch control signal. The power converter alsoincludes a means for receiving a switch signal that is derived from avoltage across the second auxiliary inductor and from the currentflowing through the primary inductor. The switch signal is used both toadjust the frequency of the inductor switch control signal such that theoutput current of the power converter remains constant and to adjust thepulse width of the inductor switch control signal such that the outputvoltage of the flyback converter remains constant. In addition, theswitch signal is used to adjust the pulse width of the inductor switchcontrol signal such that the output current of the flyback converterdoes not exceed a predetermined current limit. The controller IC ispackaged in an IC package that includes no more than three terminals.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (prior art) is a simplified schematic diagram of a constantoutput current flyback converter that is controlled on the primary sideby a controller integrated circuit with six pins.

FIG. 2 (prior art) is a simplified schematic diagram of a constantoutput current flyback converter that is controlled on the primary sideby a controller integrated circuit with four pins.

FIG. 3 is a simplified schematic diagram of a primary-side controlledflyback converter with a controller integrated circuit (IC) housed in anintegrated circuit package with only three pins in accordance with afirst embodiment of the invention.

FIG. 4 is a more detailed schematic diagram of the controller IC of FIG.3.

FIG. 5 is a flowchart of a method for controlling the output current andvoltage of the flyback converter of FIG. 3.

FIG. 6 is a diagram showing idealized waveforms that illustrate theoperation of the flyback converter of FIG. 3 while performing the methodof FIG. 5.

FIG. 7 is a diagram showing waveforms that illustrate how the flybackconverter of FIG. 3 adjusts the switching frequency so as to maintain aconstant output current and adjusts the pulse width so as to maintain aconstant output voltage.

FIG. 8 is a graph of the peak current output by the flyback converter ofFIG. 3 over time in a constant current mode and in a constant voltagemode.

FIG. 9 is a graph of output voltage versus output current for theflyback converter of FIG. 3.

FIG. 10 is a more detailed schematic diagram of an oscillator within thecontroller IC of FIG. 3.

FIG. 11 is a waveform diagram showing idealized timing waveforms of theoscillator in FIG. 10.

FIG. 12 is a more detailed schematic diagram of a current limiter withinthe controller IC of FIG. 3.

FIG. 13 is a simplified schematic diagram of a primary-side controlledflyback converter with a controller IC housed in an integrated circuitpackage with only three pins in accordance with a second embodiment ofthe invention.

FIG. 14 is a more detailed schematic diagram of the controller IC ofFIG. 13.

FIG. 15 is a flowchart of a method for controlling the output currentand voltage of the flyback converter of FIG. 13.

FIG. 16 is a diagram showing idealized waveforms that illustrate theoperation of the flyback converter of FIG. 13 while performing themethod of FIG. 15.

FIG. 17 is a simplified schematic diagram of a flyback converter with acontroller IC in a 3-pin package similar to the embodiment of FIG. 13,except with no second auxiliary inductor.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 3 is a diagram of a flyback converter 30 with a controllerintegrated circuit (IC) 31 packaged in an integrated circuit package 32.Although the term “integrated circuit” is commonly used to denote bothan integrated circuit and the integrated circuit package in which theintegrated circuit is housed, the term “integrated circuit” as usedherein denotes only the integrated circuit die. Flyback converter 30includes a transformer that converts an input voltage into a differentoutput voltage. In one embodiment, the input voltage is the voltage froma wall outlet, and the output voltage is used to charge a portableelectronic consumer device. When a main power switch in the converter isturned on, a current starts flowing through a primary inductor of thetransformer. After current ramps up through the primary inductor to apeak magnitude and is then cut, a collapsing magnetic field around theprimary inductor transfers energy to a secondary inductor. The energytransferred to the secondary inductor is output from flyback converter30 as the output current with the different output voltage. In someapplications, such as charging an electronic consumer device, it isdesirable for the output current to be maintained at a constant level aswell as prevented from exceeding a predetermined current limit.

Controller IC 31 controls the output voltage (V_(OUT)) and the outputcurrent (I_(OUT)) of flyback converter 30 by adjusting the peak currentthat flows through a primary inductor 33. The peak current is adjustedusing pulse width modulation (PWM). Controller IC 31 also controls theoutput current (I_(OUT)) of flyback converter 30 by adjusting the peakcurrent in the primary inductor and by adjusting the frequency at whichan external NPN bipolar transistor 34 is turned on and off. Transistor34 acts as the inductor switch for primary inductor 33. Controller IC 31has a power bond pad 35, a switch bond pad 36 and a ground bond pad 37.

Because current is conveyed to controller IC 31 via only three bondpads, integrated circuit package 32 has only three terminals. Eachterminal of an integrated circuit package adds cost. Thus, it is lessexpensive to produce controller IC 31 packaged in integrated circuitpackage 32 than it is to produce controller ICs requiring packages withmore than three terminals. Integrated circuit package 32 has only threeterminals: a power terminal 38, a switch terminal 39, and a groundterminal 40. Powering controller IC 31 by using a feedback signal thatcontains information indicative of the output voltage V_(OUT) and outputcurrent (I_(OUT)) when inductor switch 34 is off avoids the need forseparate terminals for (i) powering controller IC 31, (ii) providingfeedback to control the output current of flyback converter 30, and(iii) providing feedback to control the output voltage of flybackconverter 30. The four terminals used by flyback converter 22 of FIG. 2can thereby be reduced to the three terminals of flyback converter 30.

In the embodiment of FIG. 3, power bond pad 35 is connected to powerterminal 38 by a bond wire 41. Controller IC 31 receives an indicationof the output voltage V_(OUT) via power terminal 38. A feedback signal42 is received onto power terminal 38 and then travels over bond wire 41to power bond pad 35. Depending on the type of package, power terminal38 can be a lead of a low-cost TO-92 3-pin package or the lead of asmall outline transistor (SOT) package. The embodiment in whichintegrated circuit package 32 is a 3-pin TO-92 package allows controllerIC 31 to be contained in a low-cost package that is typically used tohouse a single transistor. In the embodiment of FIG. 3, switch terminal39 is connected to bond pad SW 36 by a bond wire 43. A switch signal 44is received onto switch terminal 39 and then travels over bond wire 43to bond pad SW 36.

In addition to controller IC 31, IC package 32 and inductor switch 34,flyback converter 30 also includes a transformer 45, a secondary-siderectifier 46, an output capacitor 47, a primary-side rectifier 48, astart-up resistor 49, a power capacitor (C₁) 50, and a diode 51 andresistor 52 that are coupled to the base of NPN bipolar transistor 34.Flyback converter 30 has no secondary side control circuit and nooptical coupler. A secondary side resistor 53 shown in FIG. 3 representsthe resistive loss of the copper windings of transformer 45. Transformer45 includes primary winding (inductor) 33, a secondary winding 54 and anauxiliary winding 55. Primary winding 33 of transformer 45 has Np turns;secondary winding 54 has Ns turns; and auxiliary winding 55 has Naturns. The initial start-up energy for controller IC 31 is provided bystart-up resistor 49 and power capacitor (C₁) 50. Once flyback converter30 is stable, auxiliary winding 55 of transformer 45 powers controllerIC 31 via rectifier 48.

The embodiment of flyback converter 30 shown in FIG. 3 is used inapplications requiring higher input voltage or higher power and usesexternal power-handling components, such as NPN bipolar transistor 34.Other embodiments of flyback converter 30 that are used in lower inputvoltage or lower power applications have no external bipolar transistor,MOSFET power switch or current sense circuit, all of which can beintegrated into the integrated circuit 31. In the embodiment of FIG. 3,NPN bipolar transistor 34 cooperates with controller IC 31 in an emitterswitching configuration. External NPN bipolar transistor 34 acts as aswitch to primary winding 33. In this configuration, an internal circuitin controller IC 31 drives the emitter of external bipolar transistor34. In other embodiments, to further increase the power handlingcapability and switching frequency, an external MOSFET is used as themain switch instead of bipolar transistor 34. Generally, the frequencycapability of bipolar transistor 34 is limited by the NPN basecharge/discharge time, and the high power capability of bipolartransistor 34 is limited by the base drive resistor. Thus, using bipolartransistor 34 is appropriate for applications that do not require veryhigh power or switching frequency.

FIG. 4 is a more detailed schematic diagram of controller IC 31.Controller IC 31 includes an oscillator 56, a current limiter 57,pulse-width-modulation (PWM) logic 58, a gate driver 59 and an internalmain power switch 60. In addition, controller IC 31 includes a regulatorand under-voltage lockout circuit (UVLO) 61, a reference voltagegenerator 62, a PWM error amplifier 63, an error comparator 64, afrequency modulator (FMOD) 65, a current sense amplifier 66, acompensating diode 67, a voltage divider 68, a power voltage clamp 69, asampler capacitor (C₂) 70, a first switch (SW₁) 71, a second switch(SW₂) 72, and a cord correction circuit 73.

The only feedback from the secondary side of transformer 45 used byflyback converter 30 to control the output current and voltage isfeedback from the magnetic coupling of auxiliary winding 55 andsecondary winding 54. The cost of flyback converter 30 is reduced by notusing a secondary side control circuit or an optical coupler. Inaddition, the cost of 3-pin IC package 32 is less than the cost of a4-pin package. For example, a low-cost TO-92 3-pin package typicallyused to house transistors can be used to package controller IC 31.Finally, the cost is reduced by reducing external components by placingvoltage divider 68 inside controller IC 31. The manufacturing cost offirst feedback resistor (R_(FB1)) 74 and second feedback resistor(R_(FB2)) 75 of internal voltage divider 68 is less than the cost of theexternal voltage divider resistor network 13 of flyback converter 22. Ina typical application, flyback converter 30 generates an output voltage(V_(OUT)) of about five volts. The resistors of voltage divider 68 aresized to accommodate the 5-volt output voltage. The resistance ofvoltage divider 68 is adjusted when an application requires an outputvoltage (V_(OUT)) other than five volts. For example, in order toaccommodate a 12-volt output voltage, fuses or anti-fuses or EPROM,EEPROM or other non-volatile programming means inside controller IC 31are programmed so as to adjust the voltage ratio of voltage divider 68.Using fuses, anti-fuses or other non-volatile programming means to alterthe resistance of voltage divider 68 allows controller IC 31 to beone-time-programmable (OTP).

FIG. 5 is a flowchart illustrating steps 76-83 of a method of operationof the flyback converter 30 of FIG. 3. The method controls the outputcurrent (Ion) of flyback converter 30 by adjusting the frequency of aninductor switch control signal 84 that turns main power switch 60 on andoff and indirectly inductor switch 34 on and off. The method alsocontrols the output voltage (V_(OUT)) of flyback converter 30 byadjusting the pulse width of inductor switch control signal 84 andthereby the peak current that flows through primary inductor 33 offlyback converter 30. In some applications, it is desirable for theoutput current (I_(OUT)) of flyback converter 30 to be maintained at aconstant level. The output current (I_(OUT)) is dependent on at leastthree factors: (i) the peak magnitude of an inductor current 85 throughprimary inductor 33, (ii) the inductance (L_(P)) of primary inductor 33,and (iii) the frequency (f_(OSC)) at which main power switch 60 isturned on and off by inductor switch control signal 84 allowing currentto ramp up through primary inductor 33.

The method of FIG. 5 adjusts the frequency (f_(OSC)) at which main powerswitch 60 turns on and off in order to maintain constant output current(I_(OUT)) from flyback converter 30. Thus, output current (I_(OUT)) ismaintained at a constant magnitude by adjusting either or both theswitching frequency (f_(OSC)) at which inductor current 85 ramps upthrough primary inductor 33 or the peak amount of current (I_(P))flowing through primary inductor 33.

In a first step 76 shown in FIG. 5, flyback converter 30 is connected toan input voltage (V_(IN)), and main power switch 60 is turned on. Then,inductor current 85 starts flowing through primary inductor 33. Whenmain power switch 60 is on, the voltage at the dot end of primaryinductor 33 goes low, and the voltage at the not-dot end is high. Asinductor current 85 ramps up through primary inductor 33, the inputenergy is stored in primary inductor 33. Then, the energy is transferredto secondary winding 54 when main power switch 60 is turned off. Theenergy transferred to secondary winding 54 is output from flybackconverter 30 as the output current (I_(OUT)).

In a step 77, auxiliary winding 55 is magnetically coupled to secondarywinding 54. As inductor current 85 ramps up through primary inductor 33and then stops flowing, energy is also transferred to auxiliary winding55 and generates a voltage (V_(AUX)) 86 on the dot end of auxiliarywinding 55. Voltage (V_(AUX)) 86 contains information relating to theoutput voltage when main power switch 60 is off.

In a step 78, feedback signal 42 is received onto power bond pad (VDD)35 of controller IC 31. Feedback signal 42 is derived from the voltage(V_(AUX)) 86 across auxiliary inductor 55 when auxiliary inductor 55magnetically couples with primary winding 33 and secondary winding 54.

In a step 79, feedback signal 42 is used to power controller IC 31.Regulator and under-voltage lockout circuit (UVLO) 61 receives feedbacksignal 42 from power bond pad (VDD) 35 and provides an internal powersupply to controller IC 31. In the event that the voltage (V_(DD))present on power bond pad (VDD) 35 exceeds a safe operating range, powervoltage clamp 69 acts as a protection device and dumps the excesscharge. In one embodiment, regulator 61 uses feedback signal 42 togenerate a 5-volt signal that powers the circuitry of controller IC 31,such as current limiter 57.

In steady state operation, regulator 61 receives a fifteen-volt voltagefrom feedback signal 42 onto power bond pad (VDD) 35. During start upand before any voltage is generated by auxiliary winding 55, a voltagethat is produced by start-up resistor 49 and power capacitor (C₁) 50 isreceived onto power bond pad (VDD) 35. The voltage on power capacitor(C₁) 50 builds up during startup until the under-voltage lockout turn-onthreshold of nineteen volts is reached and controller IC 31 beginsswitching main power switch 60. Regulator and under-voltage lockoutcircuit (UVLO) 61 monitors the V_(DD) voltage received as feedbacksignal 42 and enables the normal operation of controller IC 31 whenV_(DD) reaches the under-voltage lockout turn-on threshold. In thisexample, the under-voltage lockout turn-off threshold is eight volts. IfV_(DD) drops to or below the turn-off threshold, then regulator andunder-voltage lockout circuit (UVLO) 61 stops the switching ofcontroller IC 31, and charge flows through start-up resistor 49 ontopower capacitor (C₁) 50 until the under-voltage lockout turn-onthreshold of nineteen volts is again reached.

In a step 80, controller IC 31 uses feedback signal 42 to generateinductor switch control signal 84. Controller IC 31 also uses switchsignal (I_(SW)) 44 to generate inductor switch control signal 84.Controller IC 31 receives feedback signal 42 from primary-side rectifier(D₂) 48 through power terminal 38 and power bond pad (VDD) 35. Currentlimiter 57 of controller IC 31 receives switch signal (I_(SW)) 44 fromswitch bond pad 36 indicating the magnitude of inductor current 85flowing through primary inductor 33. Current limiter 57 turns off mainpower switch 60 when a predetermined peak current limit is reached.Switch signal 44 is obtained from the emitter of external NPN bipolartransistor 34 via switch terminal (SW) 39 of IC package 32. Inductorcurrent 85, which ramps up through primary inductor 33, flows throughNPN bipolar transistor 34, switch terminal 39 and switch bond pad 36.

In a step 81, inductor switch control signal 84 is asserted, whichcloses main power switch 60 and turns on inductor switch 34. Theninductor current 85 begins ramping up through primary inductor 33.Inductor switch control signal 84 has a frequency (f_(OSC)) and a pulsewidth and controls the gate of main power switch 60 through whichinductor current 85 flows. Gate driver 59 generates inductor switchcontrol signal 84 using an “N-channel on” (NCHON) signal 87. Gate driver59 is a relatively high-speed MOSFET gate driver. The inductor switchcontrol signal 84 is received by a smaller scaled internal MOSFET 88 inaddition to main power switch 60. The smaller internal MOSFET 88 and aresistor 89 form a current sense circuit. The sensed current isamplified by current sense amplifier 66 and is converted to a voltagesignal 90. Voltage signal 90 is compared by error comparator 64 to theoutput of PWM error amplifier 63.

PWM logic 58 generates the N-channel on signal 87 using a current limitsignal 91 from current limiter 57, a switching frequency signal 92 fromoscillator 56 and a pulse width signal 93 from error comparator 64.Switching frequency signal 92 provides the frequency of the pulses ofinductor switch control signal 84, and pulse width signal 93 providesthe duration of the pulse width of inductor switch control signal 84.Current limiter 57 generates current limit signal 91 using switch signal(I_(SW)) 44 and an internally generated fixed reference voltage.

In addition to limiting peak input current, flyback converter 30 alsooutputs constant current and constant voltage by operating in two modes:a constant current mode and a constant voltage mode. In constant currentmode, current limiter 57 controls the pulse width of inductor switchcontrol signal 84 such that the time (T₂) at which inductor current 85stops increasing through primary inductor 33 corresponds to the time atwhich switch signal (I_(SW)) 44 reaches a peak current limit.

In a step 82, flyback converter 30 adjusts the frequency (f_(OSC)) ofinductor switch control signal 84 using information from feedback signal42 when inductor switch 34 is turned off such that the output current(I_(OUT)) remains constant. In the constant current mode, the peak(I_(P)) of the inductor current 85 always reaches its limit, and theoutput current (I_(OUT)) is adjusted by regulating the frequency atwhich pulses of peak current ramp up through primary inductor 33.Switching frequency signal 92 output by oscillator 56 controls thefrequency (f_(OSC)) of inductor switch control signal 84 such that theoutput current (I_(OUT)) remains constant as output voltage (V_(OUT))received by the device being charged increases.

In a step 83, flyback converter 30 adjusts the pulse width of inductorswitch control signal 84 using information from feedback signal 42 wheninductor switch 34 is turned off such that the output voltage (V_(OUT))remains constant. Flyback converter 30 enters the constant voltage modewhen the load current can be satisfied with a primary-side peak currentthat is less than the predetermined peak current limit. In the constantvoltage mode when inductor current 85 is below the peak current limit,pulse width signal 93 output by error comparator 64 controls the pulsewidth of inductor switch control signal 84 such that the peak of eachpulse of inductor current 85 maintains a constant output voltage(V_(OUT)).

FIG. 6 shows idealized waveforms on various nodes of flyback converter30. The waveforms illustrate the operation of flyback converter 30during the method of FIG. 5. Main power switch 60 turns on at T₀, turnsoff at T₂, and turns on again at T₄. The time between T₀ and T₁represents the delay from when main power switch 60 is turned on andwhen inductor switch 34 turns on allowing inductor current 85 (I_(LP))to begin to ramp up. Thus, the time between T₁ and T₅ is the switchingperiod. Inductor switch 34 also exhibits a turn-off delay from time T₂to time T₂′. The time between T₁ and T₂′ is the ramp-up time. The timebetween T₂′ and T₄ is the time during which main power switch 60 isturned off. FIG. 6 illustrates that the information received fromfeedback signal 42 while main power switch 60 is turned off is used toregulate both the output current (I_(OUT)) and the output voltage(V_(OUT)). In constant current mode, current limiter 57 controls thepulse width of inductor switch control signal 84 such that the time T₂′at which inductor current 85 stops increasing through primary inductor33 corresponds to the time at which switch signal (I_(SW)) 44 reachesthe preset peak current limit.

The current waveform I_(S) shows that the current through secondarywinding 54 discharges to zero by the time T₃. FIG. 6 illustrates thatflyback converter 30 operates in a discontinuous conduction mode (DCM)because there is a gap between time T₃ at which current I_(S) stopsflowing through secondary winding 54 and the time T₅ at which inductorcurrent (I_(LP)) 85 next begins ramping up through primary inductor 33.

Feedback signal 42 provides an indication of the output voltage(V_(OUT)) of secondary winding 54. The indication of the output voltage(V_(OUT)) is used to adjust both the output voltage (V_(OUT)) and theoutput current (I_(OUT)). As shown in FIG. 3, power bond pad (VDD) 35 ofcontroller IC 31 on the primary side of transformer 45 receives theindication of the output voltage (V_(OUT)) of secondary winding 54.Feedback signal 42 on power bond pad 35 is obtained by passing thevoltage (V_(AUX)) 86 on the dot end of auxiliary winding 55 throughprimary-side rectifier (D₂) 48.

As shown in FIG. 4, a feedback voltage (V_(FB)) on node 94 of controllerIC 31 is generated by passing feedback signal 42 present on power bondpad (VDD) 35 through compensating diode 67 and voltage divider 68. Thefeedback voltage (V_(FB)) is then sampled when main power switch 60 isoff and inductor switch 34 is off. When inductor switch control signal84 is deasserted and turns off main power switch 60, a sampler switchsignal 95 is asserted and closes both first switch (SW₁) 71 and secondswitch (SW₂) 72. Then when inductor switch control signal 84 isasserted, sampler switch signal 95 opens second switch (SW₂) and samplesthe feedback voltage (V_(FB)). When inductor switch control signal 84 isasserted and main power switch 60 is on, sampler switch signal 95 alsoopens first switch (SW₁) 71. First switch (SW₁) 71 is opened primarilyin order to decrease the amount of current required in the start-upphase.

When main power switch 60 is on and first switch (SW₁) 71 is open, thevoltage (V_(AUX)) 86 goes negative, as shown in FIG. 6. The voltage offeedback signal 42 present on power bond pad (VDD) 35 is prevented,however, from going negative by primary-side rectifier (D₂) 48. Whilemain power switch 60 is on, controller IC 31 is powered by the charge onpower capacitor (C₁) 50. FIG. 6 shows the charge on power capacitor (C₁)50 while main power switch 60 is on as voltage (V_(DD)) of feedbacksignal 42. In an exaggerated manner for illustrative purposes, FIG. 6shows that the voltage (V_(DD)) on power capacitor (C₁) 50 droops whilemain power switch 60 is on as controller IC 31 consumes power. Then, attime T₂ the voltage (V_(DD)) on power capacitor (C₁) 50 is refreshedwhen sampler switch signal 95 closes first switch (SW₁) 71 and secondswitch (SW₂) 72.

The sampled feedback voltage (V_(FBS)) is held by sampler capacitor (C₂)70. The relationship between sampled feedback voltage (V_(FBS)) and theoutput voltage (V_(OUT)) is determined as follows. When inductor switch34 has just been turned off and energy is transferring to secondarywinding 54, the voltage (V_(AUX)) 86 across auxiliary winding 55 isequal to

$\begin{matrix}{V_{AUX} = {\left( {V_{OUT} + V_{D\; 1}} \right) \cdot {\frac{Na}{Ns}.}}} & (96)\end{matrix}$

The voltage (V_(DD)) of feedback signal 42 present on power bond pad(VDD) 35 equals the voltage (V_(AUX)) 86 minus the voltage drop (V_(D2))across primary-side rectifier (D₂) 48. Thus,V_(DD)+V_(D2)=(V_(OUT)+V_(D1))N_(a)/N_(s). So the voltage of feedbacksignal 42 can be expressed as

$\begin{matrix}{V_{DD} = {{\frac{N_{a}}{N_{s}} \cdot V_{OUT}} + {\left( {{\frac{N_{a}}{N_{s}} \cdot V_{D\; 1}} - V_{D\; 2}} \right).}}} & (97)\end{matrix}$

The second term is an “error” term that can be minimized by choosing aprimary-side rectifier (D₂) that has a voltage drop equal to the turnsratio N_(a)/N_(s) times the voltage drop of secondary-side rectifier(D₁) 46. Alternatively, multiple primary-side rectifier diodes can beused in series to compensate for the voltage drop of secondary-siderectifier (D₁) 46. For example, where secondary-side rectifier (D₁) 46is a Schottkey diode with a voltage drop of about 0.4 volts and theturns ratio N_(a)/N_(e) is 3:1, two 4148-type diodes each with a voltagedrop of about 0.65 volts can be used in series as the primary-siderectifiers. The “error” term would then be reduced to 0.1 volts(3·0.4V−2·0.65V).

A primary-side rectifier (D₂) 48 should be chosen that has a breakdownvoltage greater than the sum of the maximum positive voltage (V_(DD)) onpower bond pad (VDD) 35 and the peak negative voltage (V_(AUX)) 86. Forexample, where the peak of the input line voltage received by primaryinductor 33 is about 400 volts and the turns ratio N_(p)/N_(a) is 4:1,the peak negative voltage (V_(AUX)) 86 will be about −100 volts. Wherethe voltage drop across compensating diode 67 and voltage divider 68 hasbeen chosen to achieve a voltage (V_(DD)) on power bond pad (VDD) 35 ofabout fifteen volts and the maximum positive voltage (V_(DD)) isslightly greater than the clamp voltage, primary-side rectifier (D₂) 48should be chosen to have a breakdown voltage greater than one hundredtwenty volts [20V−(−100V)]. Where the under-voltage lockout turn-onthreshold is nineteen volts, the clamp voltage of power voltage clamp 69must be at least twenty volts so that a sufficient voltage level will beachieved to turn on controller IC 31.

In the embodiment of FIG. 3, compensating rectified diode (D₃) 67 withincontroller IC 31 is also used to minimize the “error” term of equation97. The voltage (V_(DD)) of feedback signal 42 present on power bond pad(VDD) 35 can also be expressed as

$\begin{matrix}{{V_{DD} = {{V_{FBS} \cdot \left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right)} + V_{D\; 3}}},} & (98)\end{matrix}$

where V_(FBS) is the sampled feedback voltage on node 99 of controllerIC 31. Combining equations 97 and 98 and solving for V_(OUT) results in

$\begin{matrix}{V_{OUT} = {{{V_{FBS}\left( \frac{N_{s}}{N_{a}} \right)}\left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right)} + {\left( \frac{N_{s}}{N_{a}} \right){\left( {V_{D\; 3} + V_{D\; 2} - {\left( \frac{N_{a}}{N_{s}} \right)V_{D\; 1}}} \right).}}}} & (100)\end{matrix}$

The “error” term can now be minimized by making the combined voltagedrop across both primary-side rectifier (D₂) 48 and compensatingrectified diode (D₃) 67 equal to the turns ratio N_(a)/N_(s) times thevoltage drop of secondary-side rectifier (D₁) 46. By choosing theappropriately sized diodes 48 and 67 that eliminate the “error” term inequation 100, the output voltage (V_(OUT)) can be adjusted based on thesampled feedback voltage (V_(FBS)) according to the followingrelationship

$\begin{matrix}{V_{OUT} = {{V_{FBS}\left( \frac{N_{s}}{N_{a}} \right)}{\left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right).}}} & (101)\end{matrix}$

Reference voltage generator 62 converts the output of regulator andunder-voltage lockout circuit (UVLO) 61 into a reference voltageV_(REF). The reference voltage V_(REF) is then summed with cordcorrection voltage (V_(CORD)) of a cord correction signal 102 generatedby cord correction circuit 73. The sum of the reference voltage VREF andthe cord correction voltage (V_(CORD)) is then compared to the sampledfeedback voltage (V_(FBS)) by PWM error amplifier 63. PWM erroramplifier 63 outputs an error signal 103. An internal compensationnetwork for PWM error amplifier 63 is formed by a resistor 104 and thecapacitors 105 and 106. Error comparator 64 receives error signal 103and voltage signal 90 and outputs pulse width signal 93. PWM logic 58receives pulse width signal 93 and uses it to adjust the pulse width ofN-channel on signal 87. Thus, error comparator 64 serves as apulse-width modulation comparator in the constant-voltage mode offlyback converter 30. When inductor current 85 is below the peak currentlimit in the constant voltage mode, the negative feedback loop ofcontroller IC 31 regulates the sampled feedback voltage (V_(FBS)) to thesum of the reference voltage V_(REF) and the cord correction voltage(V_(CORD)). Pulse width signal 93 output by error comparator 64 controlsthe pulse width of inductor switch control signal 84 such that theoutput voltage (V_(OUT)) is generated according to:

$\begin{matrix}{V_{OUT} = {\left( {V_{REF} + V_{CORD}} \right)\left( \frac{N_{s}}{N_{a}} \right){\left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right).}}} & (107)\end{matrix}$

In the constant current mode, controller IC 31 also uses informationfrom the feedback voltage (V_(FB)) on node 94 to adjust the frequency atwhich pulses of peak current ramp up through primary inductor 33.Switching frequency signal 92 output by oscillator 56 controls thefrequency (f_(OSC)) of inductor switch control signal 84 such that theoutput current (I_(OUT)) remains constant. The output current (I_(OUT))is dependent on both the switching frequency (f_(OSC)) and on the outputvoltage (V_(OUT)) because the output power of flyback converter 30 indiscontinuous conduction mode (DCM) generally depends only on the storedenergy of primary inductor 33 as follows:P _(OUT)=(V _(OUT))·I _(OUT)=½·I _(P) ² ·L _(P) ·f _(OSC)·η  (108)

where L_(P) is the inductance of primary winding 33, I_(P) is the peakcurrent through primary inductor 33, and η is the efficiency. Inconstant current mode, the peak current (I_(P)) always reaches its limitand is therefore constant. Thus, the current output (I_(OUT)) fromflyback converter 30 expressed as a function of switching frequency(f_(OSC)) and output voltage (V_(OUT)) is:

$\begin{matrix}{I_{OUT} = {\frac{\frac{1}{2} \cdot I_{P}^{2} \cdot L_{P} \cdot f_{OSC}}{V_{OUT}} \cdot {\eta.}}} & (109)\end{matrix}$

When the peak current (I_(P)) reaches its limit, then the output voltagedrops (V_(OUT)) and flyback converter 30 enters constant current mode.Equation 109 shows that when the peak current (I_(P)) through primaryinductor 33 is at its limit, the switching frequency (f_(OSC)) must beadjusted proportionally to the output voltage drops (V_(OUT)) in orderto maintain a constant output current (I_(OUT)).

Oscillator 56 obtains information on the output voltage (V_(OUT))through frequency modulator (FMOD) 65 from the feedback voltage (V_(FB))when inductor switch 34 is off. As the output voltage (V_(OUT)) receivedby the device being charged increases in the constant current mode,oscillator 56 outputs switching frequency signal 92 so as to control theswitching frequency (f_(OSC)) of inductor switch control signal 84 suchthat the switching frequency (f_(OSC)) increases proportionally theoutput voltage (V_(OUT)). Thus, in order to maintain a constant outputcurrent (I_(OUT)) while the output voltage (V_(OUT)) is increasing,controller IC 31 increases the switching frequency (f_(OSC)).

Cord correction circuit 73 receives filtered error signal 103 andgenerates cord correction signal 102 whose voltage is proportional tothat of error signal 103. Cord correction signal 102 is used to adjustthe voltage of the reference voltage (V_(REF)) to compensate for theloss of output voltage caused by the series resistance of the chargercord of flyback converter 30. Cord resistance compensation provides areasonably accurate constant voltage at the end of the cord thatconnects flyback converter 30 to the device that is to be charged orpowered, such as a cell phone or a portable media player. Output voltageis lost because the voltage at the point of load will have an I·R dropdue to the finite series resistance of the cord multiplied by the outputcurrent of the power supply. Primary-side-controlled flyback powerconverter 30 relies on the reflected feedback voltage across transformer45 from secondary winding 54 to auxiliary winding 55 to regulate theoutput voltage (V_(OUT)), but this reflected voltage does not includethe I·R voltage drop error resulting from the finite cord resistance. Inthe constant-voltage mode of operation, the output of error amplifier 63is proportional to the output current of flyback converter 30.Therefore, error signal 103 is used to produce cord correction signal102 whose voltage is proportional to output current and which is appliedto the reference voltage input of error amplifier 63 to compensate forcord resistance.

FIG. 7 is a waveform diagram showing primary inductor current (I_(LP))85, the current (I_(S)) through secondary winding 54 and feedback signal(V_(DD)) 42 over multiple switching periods (periods number 3-11) as aflyback converter 30 charges a device. The waveforms illustrate howflyback converter 30 adjusts the switching frequency (f_(OSC)) in step82 of the method of FIG. 5 so as to maintain a constant output current(I_(OUT)). As flyback converter 30 charges a device and the outputvoltage (V_(OUT)) increases in the constant current mode, the switchingfrequency (f_(OSC)) is increased so that the output current (I_(OUT))remains constant. FIG. 7 illustrates that the switching period A islonger at a lower voltage (V_(DD)) of feedback signal 42 than theswitching period B at a higher voltage (V_(DD)) of feedback signal 42.The shorter switching period B corresponds to a higher switchingfrequency (f_(OSC)).

The waveforms of FIG. 7 also illustrate how flyback converter 30 adjuststhe pulse width of inductor switch control signal 84 in step 83 of themethod of FIG. 5 so as to maintain a constant output voltage (V_(OUT)).In the constant voltage mode, controller IC 31 controls the pulse widthof inductor switch control signal 84 such that the peak of each pulse ofinductor current 85 maintains a constant output voltage (V_(OUT)). Asthe device being charged approaches a fully charged condition, theoutput voltage (V_(OUT)) approaches the predetermined maximum outputvoltage. FIG. 7 illustrates that the pulse width D is shorter than thepulse width C in order to decrease the peak current through primaryinductor 33 and thereby the output voltage (V_(OUT)) as thepredetermined limit of voltage (V_(DD)) of feedback signal 42 isreached. The negative feedback loop of controller IC 31 controls thepulse width of inductor switch control signal 84 such that the sampledfeedback voltage (V_(FBS)) equals the sum of the reference voltageV_(REF) and the cord correction voltage (V_(CORD)).

FIG. 8 is a graph of the peak current output by flyback converter 30over time in the constant current mode and constant voltage mode. Eachpeak represents the current output by flyback converter 30 during oneswitching period. Switching periods number 3-11 correspond to the samenumbered switching periods of FIG. 7. In the example of charging a cellphone battery, the charging begins in the constant current mode atperiod #1 and enters the constant voltage mode at period #9. As the cellphone battery charges and the load from the cell phone batterydecreases, the flyback converter 30 reduces the peak current throughperiod 417 in order to maintain constant voltage.

FIG. 9 is a graph of output voltage versus output current for flybackconverter 30. The numbers along the curve correspond to the peakcurrents in the periods of FIG. 8. A normal charging process begins atpoint #1 and proceeds to point #17. A fault condition occurs where theoutput voltage falls below the fault threshold represented by the dashedline. When the output voltage falls below the fault threshold, thevoltage (V_(DD)) present on power bond pad (VDD) 35 drops below theunder-voltage lockout turn-off threshold, and switching stops. Thevoltage (V_(DD)) present on power bond pad (VDD) 35 is re-charged by theinput voltage until V_(DD) reaches the turn-on threshold, switchingresumed, and flyback converter 30 re-attempts charging the battery.

FIG. 10 shows oscillator 56 of controller IC 31 in more detail.Oscillator 56 is powered by a five-volt power signal generated byregulator 61. Oscillator 56 includes a voltage comparator 110, twocurrent sources 111 and 112, and an oscillator capacitor C_(OSC) 113.Oscillator capacitor C_(OSC) 113 is charged with a charge currentI_(OSC) generated by current source 111. In this embodiment, oscillatorcapacitor C_(OSC) 113 is discharged by current source 112 at a dischargecurrent that is four times as large as the charge current. Becausecharging current source 111 is not turned off when discharging currentsource 112 is turned on, the discharging current is three times as largeas the charging current, as shown in FIG. 11. Oscillator 56 can bemodeled as an internal RC oscillator that generates a frequency f_(OSC)of switching frequency signal 92 that is dependent on the capacitance ofoscillator capacitor C_(OSC) and the oscillator resistance R_(OSC). Theoscillator resistance can be expressed as R_(OSC)=V_(FB)/I_(OSC). FMOD65 generates a bias current with a voltage that is proportional to thevoltage of feedback signal 42 when main power switch 60 is off. Currentsource 111 receives this bias current and thereby adjusts the oscillatorfrequency (f_(OSC)) based on the output voltage (V_(OUT)) of flybackconverter 30.

FIG. 12 shows current limiter 57 of controller IC 31 in more detail.Current limiter 57 includes a bias-current source 114, a comparator 115and a replica resistor (R_(REPLICA)) 116. Replica resistor (R_(REPLICA))116 replicated the drain-source resistance (R_(DSON)) of main powerswitch 60. Bias-current source 114 uses replica resistor 116 to generatea voltage on the non-inverting input lead of comparator 115corresponding to a reference current (I_(REF)). Comparator 115 thencompares the voltage of switch signal (I_(SW)) 44 to the voltagecorresponding to the reference current (I_(REF)) generated bybias-current source 114. The output of comparator 115 goes low whenswitch signal (I_(SW)) 44 exceeds the comparator threshold generated byR_(REPLICA)·I_(REF), and main power switch 60 is turned off. In constantcurrent mode, current limiter 57 controls the switch turn-off, and inthe constant voltage mode, error comparator 64 controls the switchturn-off.

FIG. 13 shows another embodiment of a flyback converter 117 with acontroller integrated circuit (IC) 118 packaged in an integrated circuitpackage 119 having only three terminals. In the embodiment of flybackconverter 30, power bond pad 35 is used both to power controller IC 31and to receive an indication of the output voltage V_(OUT). In theembodiment of flyback converter 117, however, it is switch bond pad 36that is used for multiple purposes: both to receive an indication of theoutput voltage V_(OUT) and to receive an indication of the inductorcurrent 85 flowing through primary inductor 33.

Flyback converter 117 has a second auxiliary winding 120 that enablesswitch bond pad 36 to be used to receive an indication of the outputvoltage V_(OUT). As inductor current 85 ramps up through primaryinductor 33 and then stops flowing, energy is transferred both to firstauxiliary winding 55 and to second auxiliary winding 120. A voltage(V_(AUX1)) 86 is generated on the dot end of first auxiliary winding 55,and a voltage (V_(AUX2)) 121 is generated on the dot end of secondauxiliary winding 120. First auxiliary winding 55 has N_(A1) turns, andsecond auxiliary winding 120 with N_(A2) turns. In order to ensure thatexternal NPN bipolar transistor 34 remains off when inductor switchcontrol signal 84 is deasserted and main power switch 60 is off, theturn number N_(A2) of second auxiliary winding 120 is made greater thanthe turn number N_(A1) of first auxiliary winding 55. Making N_(A2)greater than N_(A1) ensures that the voltage (V_(AUX2)) 121 present onthe emitter of inductor switch 34 when main power switch 60 is off isgreater than the voltage present on the base of inductor switch 34 thatis generated with the voltage (V_(AUX1)) 86.

An auxiliary voltage signal 122 is derived from the voltage (V_(AUX)) 86across first auxiliary inductor 55 when first auxiliary inductor 55magnetically couples with primary winding 33 and secondary winding 54.The waveform of auxiliary voltage signal 122 is substantially the sameas that of feedback signal 42 of the embodiment of flyback converter 30,except that auxiliary voltage signal 122 is not used to provide feedbackinformation to controller IC 118. The dot end of second auxiliarywinding 120 is coupled through a second primary-side rectifier 123 (D₄)to both the emitter of inductor switch 34 and to switch terminal 39.When inductor switch 34 is on and the voltage on the dot end of secondauxiliary winding 120 is negative, second primary-side rectifier 123(D₄) is reverse biased and a switch signal (V_(SW)) 124 that is receivedonto switch terminal 39 corresponds to switch signal (I_(SW)) 44 in theembodiment of flyback converter 30. When inductor switch 34 is off,switch signal (V_(SW)) 124 that is received onto switch terminal 39follows the voltage (V_(AUX2)) 121 generated by second auxiliary winding120.

As in the embodiment of flyback converter 30, the auxiliary voltagesignal 122 present on power bond pad (V_(DD)) 35 equals the voltage(V_(AUX1)) 86 minus the voltage drop (V_(D2)) across primary-siderectifier (D₂) 48. Consequently,V_(DD)+V_(D2)=(V_(OUT)+V_(D1))N_(A1)/N_(s), and the voltage of auxiliaryvoltage signal 122 provides an indication of the output voltage(V_(OUT)) of flyback converter 117 as follows

$\begin{matrix}{V_{DD} = {{\frac{N_{A\; 1}}{N_{s}} \cdot V_{OUT}} + {\left( {{\frac{N_{A\; 1}}{N_{s}} \cdot V_{D\; 1}} - V_{D\; 2}} \right).}}} & (125)\end{matrix}$

But although auxiliary voltage signal 122 provides an indication of theoutput voltage (V_(OUT)), auxiliary voltage signal 122 is used only topower controller IC 118 and to generate reference voltages in theembodiment of FIG. 13.

When main power switch 60 is off, inductor switch 34 is off, and aftercurrent has just finished ramping down to zero in secondary winding 54at time T₃, switch signal 124 similarly provides an indication of theoutput voltage (V_(OUT)) of flyback converter 117 as follows

$\begin{matrix}{V_{SW} = {{\frac{N_{A\; 2}}{N_{s}} \cdot V_{OUT}} + {\left( {{\frac{N_{A\; 2}}{N_{s}} \cdot V_{D\; 1}} - V_{D\; 4}} \right).}}} & (126)\end{matrix}$

FIG. 14 is a more detailed schematic diagram of controller IC 118.Controller IC 118 is similar to controller IC 31 except that controllerIC 118 includes a pre-amplifier 127, a feedback sampler 128 and a NORgate 129. In addition, compensating diode 67 and voltage divider 68 areconnected to switch bond pad (SW) 36 instead of to power bond pad (VDD)35.

Compensating rectified diode (D₃) 67 within controller IC 118 is used tominimize the “error” term of equation 126. The voltage (V_(SW)) ofswitch signal 124 present on switch bond pad 36 can also be expressed interms of a feedback voltage (V_(FB)) present on a node 130 as

$\begin{matrix}{V_{SW} = {{V_{FB} \cdot \left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right)} + {V_{D\; 3}.}}} & (131)\end{matrix}$

Combining equations 126 and 131 and solving for V_(OUT) results in

$\begin{matrix}{V_{OUT} = {{{V_{FB}\left( \frac{N_{s}}{N_{A\; 2}} \right)}\left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right)} + {\left( \frac{N_{s}}{N_{A\; 2}} \right){\left( {V_{D\; 3} + V_{D\; 4} - {\left( \frac{N_{A\; 2}}{N_{s}} \right)V_{D\; 1}}} \right).}}}} & (132)\end{matrix}$

The “error” term can be minimized by making the combined voltage dropacross both second primary-side rectifier 123 (D₄) and compensatingrectified diode (D₃) 67 equal to the turns ratio N_(A2)/N_(s) times thevoltage drop of secondary-side rectifier (D₁) 46. By choosing theappropriately sized diodes 123 and 67 that eliminate the “error” term inequation 132, the output voltage (V_(OUT)) can be adjusted based on thefeedback voltage (V_(FB)) on node 130 according to the followingrelationship

$\begin{matrix}{V_{OUT} = {{V_{FB}\left( \frac{N_{s}}{N_{A\; 2}} \right)}{\left( \frac{R_{{FB}\; 1} + R_{{FB}\; 2}}{R_{{FB}\; 2}} \right).}}} & (133)\end{matrix}$

Unlike the embodiment of flyback converter 30, however, the “error” termis not uniformly minimized at all times when the feedback voltage(V_(FB)) could be sampled while inductor switch 34 is off. In theembodiment of flyback converter 117, current is flowing throughcompensating diode 67 and voltage divider 68 while inductor switch 34 isoff because switch bond pad 36 is coupled to second auxiliary winding120. The voltage drop across compensating rectified diode (D₃) 67 iscurrent dependent. In contrast, in the embodiment of flyback converter30, sampled feedback voltage (V_(FBS)) is sampled at time T₄ beforecurrent begins to flow through auxiliary winding 55. Thus, in theembodiment of flyback converter 117, the feedback voltage (V_(FB)) issampled at time T₃ as current stops flowing through second auxiliarywinding 120 and immediately prior to the “free ringing” of the voltage(V_(AUX2)) 121.

Feedback sampler 128 detects when the voltage (V_(AUX2)) 121 begins toring as current stops flowing through second auxiliary winding 120. Theoutput of feedback sampler 128 is used as a control signal 134 todisconnect compensating diode 67 and voltage divider 68 from switch bondpad (SW) 36 at time T₃ when voltage (V_(AUX2)) 121 begins to ringbecause there is a potential that voltage (V_(AUX2)) 121 minus thevoltage drop across second primary-side rectifier 123 (D₄) could fallbelow the voltage on the base of inductor switch 34 and turn on switch34. When control signal 134 is asserted, the voltage of switch signal(V_(SW)) 124 rises to near the auxiliary voltage signal (V_(DD)) 122present on power bond pad (VDD) 35.

When an insignificant amount of current is flowing through compensatingdiode 67, and appropriately sized diodes 123 and 67 have been chosen toeliminate the “error” term in equation 132, then the feedback voltage(V_(FB)) on node 130 provides an indication of the output voltage(V_(OUT)) according to equation 133. The feedback voltage (V_(FB)) onnode 130 is compared to the sum of the reference voltage V_(REF) and thecord correction voltage (V_(CORD)) to produce an error signal, which isamplified by pre-amplifier 127, sampled by feedback sampler 128, and fedto PWM error amplifier 63. In a manner similar to flyback converter 30,the negative feedback loop of controller IC 117 regulates the feedbackvoltage (V_(FB)) on node 130 to the sum of the reference voltage V_(REF)and the cord correction voltage (V_(CORD)). In the constant voltagemode, the feedback voltage (V_(FB)) on node 130 is regulated byadjusting the pulse width of inductor switch control signal 84 such thatthe output voltage (V_(OUT)) remains constant.

In a manner similar to flyback converter 30, the output current is alsoregulated. As indicated by equation 109 above, the current output(I_(OUT)) from flyback converter 117 is proportional to the switchingfrequency (f_(OSC)) and inversely proportional to the output voltage(V_(OUT)). In the constant current mode while a device is being chargedand the output voltage (V_(OUT)) is increasing, controller IC 118increases the switching frequency (f_(OSC)) at the same rate thatV_(OUT) increases in order to maintain a constant output current(I_(OUT)). To adjust the switching frequency (f_(OSC)), oscillator 56obtains information on the output voltage (V_(OUT)) through frequencymodulator (FMOD) 65 from the feedback voltage (V_(FB)) on node 130 attime T₃.

Flyback converter 117 also adjusts peak current in a manner similar tothat used by flyback converter 30. Current limiter 57 of controller IC118 receives switch signal (V_(SW)) 124 from switch bond pad 36indicating the magnitude of inductor current 85 flowing through primaryinductor 33. When the current of switch signal (I_(SW)) 44 exceeds thepredetermined peak current limit, comparator 115 of current limiter 57trips and turns off main power switch 60.

FIG. 15 is a flowchart illustrating steps 135-141 of a method ofoperation of the flyback converter 117 of FIG. 13.

In a step 135, second auxiliary inductor 120 is coupled to secondaryinductor 54 flyback converter 117.

In a step 136, switch signal (V_(SW)) 124 is derived from the voltage(V_(AUX2)) 121 across second auxiliary winding 120 and is received ontoswitch bond pad 36.

In a step 137, controller IC 118 generates inductor switch controlsignal 84 using switch signal (V_(SW)) 124.

In a step 138, main power switch 60 is turned on and off using inductorswitch control signal 84.

In a step 139, controller IC 118 uses switch signal (V_(SW)) 124 toadjust the pulse width of inductor switch control signal 84 such that apredetermined current limit of the output current (I_(OUT)) of flybackconverter 117 is not exceeded. The predetermined current limit isdefined according to the requirements of the device being charged.

In a step 140, controller IC 118 uses switch signal (V_(SW)) 124 toadjust the frequency (f_(OSC)) of inductor switch control signal 84 suchthat the output current (I_(OUT)) of flyback converter 117 remainsconstant in the constant current mode.

In a step 141, controller IC 118 uses switch signal (V_(SW)) 124 toadjust the pulse width of inductor switch control signal 84 such thatthe output voltage (V_(OUT)) of flyback converter 117 remains constantin the constant voltage mode.

FIG. 16 shows idealized waveforms on various nodes of flyback converter117. The waveforms illustrate the operation of flyback converter 117during the method of FIG. 15. Main power switch 60 turns on at T₀, turnsoff at T₂, and turns on again at T₄. The time between T₀ and T₁represents the delay from when main power switch 60 is turned on andwhen inductor switch 34 turns on allowing inductor current 85 (I_(LP))to begin to ramp up. The time between T₁ and T₂′ is the ramp-up time.The time between T₂′ and T₄ is the time during which main power switch60 is turned off. FIG. 16 illustrates that the information received fromswitch signal (V_(SW)) 124 while main power switch 60 is turned off isused to regulate both the output current (I_(OUT)) and the outputvoltage (V_(OUT)). Current limiter 57 controls the pulse width ofinductor switch control signal 84 such that the time T₂ at whichinductor current 85 stops increasing through primary inductor 33corresponds to the time at which switch signal (V_(SW)) 124 reaches thepreset peak current limit.

The current waveform I_(S) shows that the current through secondarywinding 54 discharges to zero by the time T₃. At time T₃ at whichcurrent I_(S) stops flowing through secondary winding 54, switch signal(V_(SW)) 124 provides an indication of the output voltage (V_(OUT)) ofsecondary winding 54. The indication of the output voltage (V_(OUT)) isused to regulate both the output current (I_(OUT)) when the load demandsan output current that is above the constant current limit and theoutput voltage (V_(OUT)) when the output current is below the constantcurrent limit.

FIG. 17 shows yet another embodiment of a flyback converter 142 withcontroller integrated circuit (IC) 118 packaged in an integrated circuitpackage 119 having only three terminals. Flyback converter 142 has onlytwo three inductors and no second auxiliary inductor. The embodiment ofFIG. 17 is similar to the embodiment of FIG. 13 except that switchterminal 39 is coupled through second primary-side rectifier 123 (D₄) tofirst auxiliary winding 55 instead of to a second auxiliary winding. Inthe embodiment of FIG. 17, the voltage (V_(AUX2)) 121 present on theemitter of inductor switch 34 is equivalent to the voltage (V_(AUX)) 86across first auxiliary inductor 55.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Although pulse-width-modulation (PWM)logic 45 is described above as employing pulse width modulation in thegeneration of NCHON signal 87 and inductor switch control signal 84,variable frequency modulation can be used as an alternative to fixedfrequency PWM. In alternative embodiments, variable-frequency pulsefrequency modulation (PFM) is used to generate NCHON signal 87 andinductor switch control signal 84. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

1. An integrated circuit package, comprising: a power terminal thatreceives an auxiliary voltage signal, wherein the auxiliary voltagesignal is derived from a voltage across a first inductor of a flybackconverter, and wherein the auxiliary voltage signal provides power to acontroller integrated circuit contained in the integrated circuitpackage; a ground terminal through which the controller integratedcircuit is grounded; and a switch terminal that receives a switch signaland that is coupled to an inductor switch, wherein the inductor switchis turned on by an inductor switch control signal that has a frequency,wherein the switch signal is used by the controller integrated circuitto generate the inductor switch control signal, wherein the controllerintegrated circuit adjusts the frequency such that an output current ofthe flyback converter remains constant, and wherein the integratedcircuit package has only three terminals, those being the powerterminal, the ground terminal and the switch terminal.
 2. The integratedcircuit package of claim 1, wherein the switch signal is indicative of arate at which current flows through a second inductor of the flybackconverter.
 3. The integrated circuit package of claim 2, wherein thefirst inductor is an auxiliary inductor of the flyback converter, andwherein the second inductor is a primary inductor of the flybackconverter.
 4. An integrated circuit package, comprising: a powerterminal that receives an auxiliary voltage signal, wherein theauxiliary voltage signal is derived from a voltage across a firstinductor of a flyback converter, and wherein the auxiliary voltagesignal provides power to a controller integrated circuit contained inthe integrated circuit package; a ground terminal through which thecontroller integrated circuit is grounded; and a switch terminal thatreceives a switch signal and that is coupled to an inductor switch,wherein the inductor switch is turned on by an inductor switch controlsignal that has a frequency, wherein the switch signal is used by thecontroller integrated circuit to generate the inductor switch controlsignal, wherein the controller integrated circuit adjusts the frequencysuch that an output current of the flyback converter remains constant,wherein the integrated circuit package includes no terminals other thanthe power terminal, the ground terminal and the switch terminal, whereinthe first inductor is a first auxiliary inductor, wherein the inductorswitch is coupled through a transistor to a primary inductor, andwherein the flyback converter includes the primary inductor, a secondaryinductor, the first auxiliary inductor and a second auxiliary inductor.5. The integrated circuit package of claim 1, wherein the switchterminal is taken from the group consisting of: a pin of a transistoroutline (TO) package and a pin of a small outline transistor (SOT)package.
 6. The integrated circuit package of claim 1, wherein aninductor current flowing through the first inductor reaches a peakcurrent, and wherein the controller integrated circuit controls the peakcurrent such that the output current of the flyback converter does notexceed a predetermined current limit.
 7. The integrated circuit packageof claim 1, wherein an inductor current flowing through the firstinductor reaches a peak current, wherein the inductor switch controlsignal has a pulse width, and wherein the controller integrated circuitadjusts the pulse width to control the peak current such that an outputvoltage of the flyback converter remains constant.
 8. A power converter,comprising: a primary inductor; an auxiliary inductor that ismagnetically coupled to the primary inductor; and a controllerintegrated circuit having an inductor switch, a power bond pad, a switchbond pad and a ground bond pad, wherein the inductor switch is coupledto the switch bond pad and is turned on by an inductor switch controlsignal that has a frequency, wherein the controller integrated circuitreceives power through the power bond pad, wherein the switch bond padreceives a switch signal that is used by the controller integratedcircuit to generate the inductor switch control signal, wherein thecontroller integrated circuit has only three bond pads, those being thepower bond pad, the switch bond pad and the ground bond pad, and whereinthe controller integrated circuit adjusts the frequency of the inductorswitch control signal in a constant current mode such that an outputcurrent of the power converter remains constant.
 9. The power converterof claim 8, wherein the controller integrated circuit is packaged in anintegrated circuit package taken from the group consisting of: atransistor outline (TO) package and a small outline transistor (SOT)package.
 10. The power converter of claim 8, wherein the inductor switchcontrol signal has a pulse width, wherein the switch signal is derivedfrom a voltage across the auxiliary inductor, and wherein the controllerintegrated circuit uses the switch signal to adjust the pulse width suchthat an output voltage of the flyback converter remains constant.
 11. Apower converter, comprising: a primary inductor; an auxiliary inductorthat is magnetically coupled to the primary inductor; and a controllerintegrated circuit having an inductor switch that is coupled to a switchbond pad, wherein the inductor switch is turned on by an inductor switchcontrol signal that has a frequency and a pulse width, wherein theswitch bond pad receives a switch signal that is used to generate theinductor switch control signal, wherein the switch signal is derivedfrom a voltage across the auxiliary inductor, wherein the controllerintegrated circuit uses the switch signal to adjust the frequency suchthat an output current of the power converter remains constant, whereinthe controller integrated circuit uses the switch signal to adjust thepulse width such that an output voltage of the power converter remainsconstant, wherein the controller integrated circuit uses the switchsignal to adjust the pulse width such that a predetermined current limitis not exceeded, wherein the controller integrated circuit is packagedin an integrated circuit package that has only three terminals, thosebeing a switch terminal, a power terminal and a ground terminal, whereinthe switch terminal is coupled to the switch bond pad, and wherein thepower terminal is coupled to the power bond pad.
 12. The power converterof claim 11, wherein the integrated circuit package is taken from thegroup consisting of: a transistor outline (TO) package and a smalloutline transistor (SOT) package.
 13. The power converter of claim 11,wherein the power converter is a flyback converter.
 14. A methodcomprising: magnetically coupling an auxiliary inductor to a secondaryinductor of a power converter, wherein the power converter has acontroller integrated circuit, and wherein the controller integratedcircuit has an inductor switch; receiving a switch signal onto a switchbond pad of the controller integrated circuit, wherein the switch signalis derived from a voltage across the auxiliary inductor; generating aninductor switch control signal using the switch signal, wherein theinductor switch control signal has a frequency and a pulse width;turning on the inductor switch using the inductor switch control signal;adjusting the frequency of the inductor switch control signal using theswitch signal such that an output current of the power converter remainsconstant; and adjusting the pulse width of the inductor switch controlsignal using the switch signal such that an output voltage of the powerconverter remains constant, wherein the controller integrated circuit iscontained in an integrated circuit package that has only threeterminals, those being a power terminal, a switch terminal and a groundterminal, wherein the controller integrated circuit has a power bond padand a ground bond pad, wherein the power terminal is coupled to thepower bond pad, the switch terminal is coupled to the switch bond padand the ground terminal is coupled to the ground bond pad.
 15. Themethod of claim 14, further comprising: adjusting the pulse width of theinductor switch control signal using the switch signal such that apredetermined current limit is not exceeded.
 16. The method of claim 14,wherein the power converter has a second auxiliary inductor, furthercomprising: magnetically coupling the second auxiliary inductor to thesecondary inductor; receiving an auxiliary voltage signal onto a powerbond pad of the controller integrated circuit, wherein the auxiliaryvoltage signal is derived from a voltage across the second auxiliaryinductor; and powering the controller integrated circuit using theauxiliary voltage signal.
 17. The method of claim 14, whereininformation conveyed in the switch signal while the inductor switch isturned off is used to adjust the frequency of the inductor switchcontrol signal such that the output current of the power converterremains constant, and wherein information conveyed in the switch signalwhile the inductor switch is turned off is used to adjust the pulsewidth of the inductor switch control signal such that the output voltageof the power converter remains constant.
 18. The method of claim 14,wherein the integrated circuit package is taken from the groupconsisting of: a transistor outline (TO) package and a small outlinetransistor (SOT) package.
 19. A power converter, comprising: a primaryinductor; an auxiliary inductor that is magnetically coupled to theprimary inductor, wherein an inductor switch of a controller integratedcircuit is turned on by an inductor switch control signal that has afrequency and a pulse width; a ground bond pad through which thecontroller integrated circuit is grounded; a power bond pad throughwhich the controller integrated circuit is powered; and means forreceiving a switch signal that is used to generate the inductor switchcontrol signal, wherein the switch signal is derived from a voltageacross the auxiliary inductor, wherein the means is coupled to theinductor switch, wherein the switch signal is used both to adjust thefrequency such that an output current of the power converter remainsconstant and to adjust the pulse width such that an output voltage ofthe flyback converter remains constant, wherein no current is conveyedto or from the controller integrated circuit except through the groundbond pad, the power bond pad and the means, and wherein the controllerintegrated circuit is contained in an integrated circuit package thathas only three terminals, those being a power terminal, a switchterminal and a ground terminal.
 20. The power converter of claim 19,wherein the power converter is a flyback converter.
 21. The powerconverter of claim 19, further comprising: a ground bond pad throughwhich the controller integrated circuit is grounded; and a power bondpad through which the controller integrated circuit is powered, whereinthe means is coupled to the switch terminal, wherein the ground bond padis coupled to the ground terminal, and wherein the power bond pad iscoupled to the power terminal.
 22. The power converter of claim 19,wherein the switch signal is received onto the controller integratedcircuit only via the means.